Full color printing has become a desired goal of office products. One type of full color printer which has significant potential for fulfilling such a goal is the ink jet printer. In one common design of such printers, a reservoir of liquid ink is connected to an ink output orifice via a capillary tube. In the tube, a heater element is provided, responsive to an on/off or binary printing signal. When printing is required and a printing signal is directed to the heater element, the heater element rapidly heats ink in the capillary tube adjacent thereto to a gaseous state, producing a pressure differential which expels a droplet of ink from the orifice, directing the droplet to a sheet of receiving material, such as paper. Color printing is accomplished by providing multiple layers or separations of ink on the page. Commonly, colors are provided by subtractive combinations of cyan, magenta and yellow inks. To print black, a combination of equal amounts of cyan, magenta and yellow is printed, or a fourth black ink is used as a substitute. Undercolor removal, a well known process in the printing arts, can be used to print a single layer of black ink as a substitute for the combination of equal amounts of cyan, magenta and yellow. For a fuller discussion of under color removal and its application to electronically derived or created images, reference is made to J.A.C. Yule, Principles of Color Reproduction, (John Wiley & Sons, Inc., New York, 1967), pages 294-327. Other full color printing processes may use dry powder or liquid toners.
A problem of ink jet printers is that the liquid inks used have a finite drying time, which tends to be somewhat longer than desirable. Further, the drying time of any particular area is at least partly a function of the amount of ink deposited on that area. While satisfactory drying times are possible with black-only or single separation printing, once multiple separations are required, the large amount of liquid on the page causes the problems of ink puddling or pooling, bleeding to adjacent image areas, and flow through to the back side of the receiving material. Paper cockle is also a problem due to saturation of the paper receiving material and subsequent rapid drying. Particularly, problems are noted in the printed image at high ink coverage areas, and high coverage areas where high contrast image edges occur. While certain materials variations, such as different inks or special papers may resolve some of these problems, each brings its own distinct problems to the process. While special treated papers optimized for ink jet use are possible, plain papers are preferred from cost and convenience standpoints.
Using one available set of materials, a maximum ink coverage of about 150% is required for printing without artifacts resulting from too much ink. As used herein, ink coverage refers to the number of ON pixels in a region for all the separations, divided by the total number of pixels in the the region in one separation. Without undercolor removal, typical full color image may require ink coverage in the range of 200-300%. With undercolor removal, maximum ink coverage may be down to 200%, but no lower. Additionally, it would be highly desirable for the process to operate irrespective of image content, or on the separation binary bitmaps without further image information.
While ink jet printing has a notable problem with the case of high ink coverage, other printing processes which are printed on a pixel by pixel basis also have problems with excessive marking material. Notably, electrophotographic printing methods using a pixel by pixel printing process for forming a latent image for development by dry or liquid toner marking materials, can suffer from excessive marking material, evidenced by sheet cockling, and curling caused by differential shrinkage of toner and paper in the printing process.
U.S. Pat. No. 4,930,018 to Chan et al. teaches the reduction of paper cockle and graininess of ink jet prints. Printing of a given scan line occurs multiple times, with three different dye loadings, with pixels requiring the highest dye loading printed on one pass, pixels requiring an intermediate dye loading printed on another pass, and pixels requiring the lowest dye loading on another pass. The method takes as input continuous tone RGB (red--green--blue) images and performs RGB-CMYK (cyan--magenta--yellow key or black) conversion with full under color removal. As understood, printing is performed at half resolution, so that "pixels" in the input image correspond to 2.times.2 blocks in the output image. The image data is first error diffused from 8 bits per pixel per separation to 4 bits pixel per separation. Then, for each pixel, a count of up to 4 drops of each dye loading is computed, for each separation. There are multiple choices, ranked in order of total ink coverage. If the highest coverage choice exceeds the maximum allowable coverage, the separation with highest coverage is changed to use a lower coverage value for the same gray level, if possible. If it is not possible to stay at the same gray level, the gray level for that separation is dropped by one, and the error passed on to neighbors. The process iterates until the total ink coverage is as low as required. Pixels within the 2.times.2 block are assigned values (0 or 1) by proceeding around the block in clockwise order, and filling in pixels in order. First, the high dye load pixels are turned on, then the medium, then the low. Within each dye loading group, first black is turned on, until there are no more black pixels of that dye loading, then the next pixels in the cycle are cyan, until there are no more cyan required, then magenta, and yellow, and then the next dye load group. By maximizing ink coverage and using multiple dye loadings, they reduce the noisiness of the image, and by maintaining the total ink coverage within known limits, they prevent the many problems associated with excessive ink.
U.S. Pat. No. 4,999,646 to Trask teaches limiting coverage to 100% coverage (by the above definition of coverage), or perhaps between 100 and 200% coverage (if 100% corresponds exactly to no white spaces on a page), owing to the circular shape and overlap of print dots. Coverage is limited by using 2.times.2 super pixels and assigning each one drop per pixel in a combination that depends on the color required. Assuming one bit per separation input with full undercolor removal, there are eight possible colors that could be requested (including white). In order to reduce patterning due to the multiple swaths, two passes are used, each of a checkerboard pattern of pixels (the two passes being offset to provide full coverage). The two pass process allows ink to dry between passes.